Bone is a classic example of excellent natural material engineering. It primarily consists of tropocollagen fibrils – which would be too soft to support the weight of the skeleton under its daily loads – and hydroxyapatite, a stiff but fragile material prone to fracture. However, the alliance of these two imperfect candidates is an extremely tough, lightweight and robust material.

Based on a simple molecular model of mineralized collagen fibrils, Buehler showed that, as might be expected, the stiffness of mineralized fibrils lies somewhere between the two extremes of the component materials, with as more recent studies reveal, the mineral components bearing up to four times the stress of the collagen fibrils. However, in addition his 2007 study pointed out that the mineralization increases the energy dissipation during deformation. As he explains in his report, “The fibrillar toughening mechanism increases the resistance to fracture by forming large local yield regions around crack-like defects, a mechanism that protects the integrity of the entire structure by allowing for localized failure.”

Markus Buehler and colleagues describe what makes spider webs so strong.

Strength through compromise

Spider silk is another material that invokes give and compromise at nanoscale dimensions to achieve miraculous strength at the macroscale. Hydrogen-bonded beta sheets in the spider silk are embedded in a glycine-rich semi-amorphous protein structure – the main component of the material. The hydrogen bond interactions between the sheets are weak but they work together, and under stress they readily break and reform, accommodating a slip-stick mechanism that makes spider silk incredibly resilient to fracture. As Buehler and Su explain in their review, “Cooperative deformation of hydrogen bonds transforms weakness into strength within the crystalline regions.”

The stick-slip mechanism of beta sheets also underpins the observed resilience to defects so that it can withstand fractures that span 50% of a fibril’s width or length. Here the nanoscale of the structures is key. For fibrils narrower than 50–80nm the stresses and strains are evenly distributed, so the fibre behaves as if it were defect-free; above these critical dimensions, the stress and strain experienced begins to increase.

Molecular tuning of mechanical properties

The beta sheets and other protein structures that give rise to the impressive macroscopic mechanical properties of spider silk are in turn determined by the complex molecular structure of amino acid sequences. As an example, Su and Buehler break down the components of dragline silk, which spiders produce to escape predators.

The two main proteins – major ampullate dragline silk protein 1 (MaSp1) and major ampullate dragline silk protein 2 (MaSp2) – differ in terms of the quantity of proline protein residue and the main repeats. Both contain poly-Ala and poly-Gly-Ala – where the polyalanine domains form stiff β-sheet nanocrystals responsible for the silk’s strength. However, in addition MaSp1 contains GGX motifs – which fold into 310-helices – and MaSp2 contains GPGXX, which forms β-spirals producing the semi-amorphous and disordered domain responsible for silk’s elasticity. The structures these proteins form can thus be controlled by modifying these sequences, such as increasing the number of polyalanine domains to form silk with more β-sheet crystals, and consequently a stronger fibre.

Guest editor Raul Perez-Jimenez describes protein folding research at CIC nanoGUNE

The role of protein folding

The structures of proteins are crucial to all their biological functions, and understanding how protein chains fold into their three-dimensional configurations has been the focus of academic research and citizen science projects to crowd source computational power. The aim is to gain invaluable insights into the causes and potential remedies for diseases including Alzheimer's, "Mad cow" (BSE), CJD, ALS, AIDS, Huntington's, and Parkinson's disease. The Nanotechnology Focus on Protein Folding provides a collection of some of the latest research pushing the frontiers in this field.

Despite the importance of protein conformation, the crowded cell environment can cause aggregation into toxic species and make it difficult for proteins to reach the target morphology, as Maria Rosario Fernández-Fernández, Begoña Sot and José María Valpuesta from the Universidad Autonoma de Madrid describe in their review in the collection. “To avoid this, nature has developed protein quality-control mechanisms that include a complex surveillance system to guide protein homeostasis, or proteostasis, which is carried out by a large, diverse group of proteins termed molecular chaperones.” They go on to describe the structure and function of some of these chaperones, and summarize some of the efforts to exploit them in nanotechnological applications.

Stiff approach to cancer diagnosis

One of the proteins known to have a significant effect on both structural and functional elements that maintain cell morphology and adhesion, among other characteristics, is cytoskeletal actin. It is found in all eukaryotic cells, and alteration in actin remodelling has been linked to malignant cellular characteristics associated with cancer, where the mechanical properties of tissue changes. In fact the differences between cancerous and non-cancerous tissue are so distinct that Marija Plodinec, Rod Lim and colleagues at the University of Basel, University Hospital Basel and the Friedrich Miescher Institute for Biomedical Research in Switzerland have shown that a stiffness profile of breast tissue as measured using an atomic force microscope (AFM) can effectively distinguish the malignant from the healthy. Lim and his colleagues have exploited this in the development of an AFM-based device called ARTIDIS (automated and reliable tissue diagnostics) for cancer diagnosis.

Rod Lim at Basel University describes the AFM-based ARTIDIS technique for diagnosing breast cancer

Noting the reported health benefits of green tea extract (GTE) and its touted potential for fighting cancer, James K Gimzewski at the University of California in Los Angeles, US (UCLA) and the National Institute of Materials Science in Japan (NIMS) and colleagues studied the stiffness of clinically derived metastatic cancer cells and normal mesothelial cells both before and after treatment with green tea extract (GTE). Alongside Sarah E Cross, Yu-Sheng Jin, Qing-Yi Lu and JianYu Rao at UCLA, he used AFM to monitor the cells’ mechanical properties and found that while the cancer cells had a lower Young’s modulus, GTE treatment increased it to a stiffness similar to the normal cells.

Further proteomics and fluorescence imaging studies linked the changes to the protein actin, revealing GTE-induced alterations in the actin binding protein (ABP) annexin-I. The researchers report on the results, “These findings confirm the functional and observed mechanical affect associated with GTE-induced annexin-I expression on stimulating actin polymerization,” and conclude GTE may offer a promising and less debilitating alternative to other chemotherapies.

Collagen fibrils, actin and beta sheets are just a few examples of nanostructures and proteins with key structural functions that lead to observable macroscale mechanical properties. As with so many hierarchical systems the interactions between the proteins and nanostructures underpin the macroscopically observed behaviour. In the words attributed to Aristotle, “the whole is more than the sum of its parts”. Compromise and cooperation between the constituent components are key to many of the natural materials that have superlative mechanical properties, and progress in understanding how to exploit these relationships may be a gold mine for materials science.